Investigating the effect of global warming on subtropical high pressure

Introduction Climate change in recent years has led to changes in atmospheric patterns and the appearance of climatic anomalies in most parts of the world. Earth’s climate is a complex dynamic system that involves hydrosphere, cryosphere, biosphere and lithosphere. If any of these system changes, other systems will quickly or slowly align themselves with it, and the outcome of this coordination also affects the system initiated the change. Eventually, an endless chain of links is created between these systems. The interaction between these four systems is responsible for the concern of weather and climate scientists in recent years and propounded the term of "climate change". The result of global warming is climate change. The process of climate change, especially temperature changes, is one of the most important discussions in the field of environmental sciences. Many of our age-old problems, such as floods, storms, droughts, changes in atmospheric patterns, and so on, are rooted in climate change, especially in air temperature increasing. This study aimed to investigate the effect of global warming on subtropical high pressure behavior. Data and Methodology In order to investigate the effect of global warming on the subtropical high pressure behavior, the following steps have been taken. In the first step, maximum daily temperature data of 49 synoptic stations during the period from 1977 to 2016 were used to study the frequency of temperature higher than percentile 95 in each year. Regarding the frequency of temperature higher than percentile 95, this trend has been dramatically has been rose in 1996, so this year has been set as the border between the two pre-warming and post-warming periods. In the second stage, given that global warming is expected to increase these extreme temperature, frequency of temperature higher than percentile 95 was investigated in both periods. In the third stage, changes in the behavior of subtropical high-pressure in terms of height and spatial extent were determined based on 500-hPa geopotential data, derived from European Center for Medium – Range Weather Forecasts (ECMWF). Finally, to prove the existing relationship between the data, the 500-hPa geopotential height anomalies were plotted over the two periods and analyzed to determine that what changes occured in height of the middle level. Result and Discussion By examining the results, it was found that the long-term average of core height of the subtropical high pressure during the second period (1996-1997) has increased by 10 meters relative to the first period (1996-2016). Given the frequency of the thresholds of percentile 95 of the second period, it can be said that most stations have experienced extreme temperatures, so it can be said that global warming has been proven. It can be said that during the current period, a temperature of 40°C is a normal temperature. Therefore, due to the mutual and direct relationship between temperature and height of the atmosphere, it can be said that the reason for increasing the height of the core of the subtropical high pressure is the increase of temperature in the lower layers of the atmosphere. When the temperature increases in the layers near the land surface, thermal low pressure on the land surface and the dynamical high pressure resulting from the subtropical high pressure subsidence and some systems, including Monsoon, are created and for every 1000 meters, while increasing power, it increases air temperature by 6˚C. Thus, the core height of the subtropical high pressure is increased and the maximum temperatures are recorded, especially during the warm months. Pearson correlations also indicate a very strong and positive correlation between the core height of the subtropical high pressure and the maximum temperature in the both periods. Conclusion The results of the analysis of the maximum temperature data showed that during the first period, the temperature reached 30.5 ° C in percentile 95 while during the second period with 1˚C increase it reached 40.5˚C. It can be said that in the first period of global warming we did not have much intensity in our country, but in the second period temperature reached its maximum and the effect of this warming is seen in the recorded temperature. In other words, global warming has been proven and the frequency of temperatures above 40.5 ˚ C has become prevalent in most stations. Spatial analysis of the core of subtropical high pressure indicates that its highest height in the first period over Iran is 5910 m which affect fewer stations. But in the second period, the core height of subtropical high pressure is 5940 meters, which, in comparison with the first period, shows increase in both height and extent resulted in higher temperature. It was found that the long-term average height of the subtropical high pressure core during the second period (1996-1997) has increased by 10 meters relative to the first period (1996-2016). Given the frequency of the thresholds of percentile 95 of the second period, it can be said that most stations have experienced extreme temperatures, so it can be said that global warming has been proven. In other words, during the current period, the temperature of 40 degrees is a normal temperature.